1. Field of the Invention
The present invention relates to a zoom lens system that is suitable for use with a compact camera and which has a smaller degree of constraint on back focus than when it is used with a single-reflex lens camera.
2. Background of the Invention
The following three types of zoom lens system have heretofore been proposed for use with compact cameras. The first type is a telephoto zoom lens composed of two lens groups, the first group having a positive focal length inclusive of a stop diaphragm and the second lens group having a negative focal length. Examples of this first type are shown in Unexamined Published Japanese Patent Application Nos. 128911/1981, 201213/1982, 48009/1985, 170816/1985 and 191216/1985. The second type is a modification of the first type and is a composed of three lens groups. The first group has a positive focal length inclusive of a stop diaphragm. The second lens group also has a positive focal length and the third lens group has a negative focal length. An example of this second type is shown in Unexamined Published Japanese Patent Application No. 184916/1983. The third type is composed of four lens groups, the first group having a positive focal length, the second group having a negative focal length, the third group having a positive focal length and the fourth group having a negative focal length. An example of this third type is shown in Unexamined Published Japanese Patent Application No. 57814/1985.
However, these three types of conventional zoom lens system adapted to compact cameras have their own problems as described below. The first type which is composed of two lens groups features a very simple lens configuration but the first and second lens groups must be displaced by great amounts during zooming. The second type is a modification of the first type in that the first lens group is divided into two groups, each having a positive focal length and the amount by which each of the lens groups must displaced during zooming is no smaller than in the first type. The third type which is composed of four lens groups has the advantage that the individual lens groups need to be displaced by smaller amounts than in the first and second types. However, this type of lens system is difficult to manufacture for two reasons. First, its lens arrangement is complex. Secondly, the four lens groups, particularly the second and third lens groups, have such great powers that poor workmanship can cause substantial effects on the performance of the final product.
In practical applications, the zoom lens systems of all three types have one common problem in that, being intended for use with compact cameras, their design does not include any constraint on back focus. In an extreme case, the back focus is as small as about 1 mm. The absence of any constraint on back focus is advantageous for the purpose of reducing the overall length of the system but, on the other hand, the lens diameter of the rear group is increased and the overall size of the camera including the mechanisms of zooming, auto-focusing and other capabilities will by no means be smaller than when certain constrains on back focus exist. Another problem associated with the absence of any constraints on back focus is that it increases the chance of internal reflections occurring between the film surface and the surfaces of lenses in the rear group.
In order to solve these problems of the prior art zoom lens systems for use with compact cameras, the applicant of the present application previously accomplished an invention which is described in Japanese Patent Application No. 232459/1985 (hereinafter referred to as the prior application). According to the embodiments shown in the specification of the prior application, the claimed system is capable of attaining zoom ratios of the order of 1.6. However, there still exists the need of consumers generally for a zoom lens having a greater zoom ratio. If an attempt is made to attain a larger zoom ratio by the lens configurations shown in the embodiments of the prior application, the amounts of aberrations such as coma aberration and distortion are increased. Therefore, there is room for improvement in the proposal of the prior application in order for it to satisfy the consumer's need for attaining a zoom lens system having a zoom ratio greater than 1.6.